1. Field of the Invention
The present invention generally relates to an optical memory and an apparatus for recording and reading an optical memory and, more particularly, to an improved optical memory and an apparatus for recording and reading data quickly and accurately.
2. Description of the Related Art
Optical memories having a layer or film of semiconductor materials are widely used in industrial and consumer recording systems such as video recorders, digital audio recorders, and document digital recorders. A signal is recorded on the optical disc by irradiation with a laser beam to form bits of information or data in the layer of semiconductor materials. The optical disc permits direct reading after the information has been recorded and random access to the information. Optical discs may also be used for erasable recording.
There are different techniques by which information may be recorded using optical discs. These techniques include ablation, topography change, chemical reaction, particle coalescence, phase change, and magnetization change. For the first five techniques, the optical contrast is obtained by the difference in reflectivity between the recorded and unrecorded states. In magnetization change, the optical contrast is obtained from the change in the direction of the polarization of light due to the change in the polarity of magnetization in a recorded area.
These recording techniques are known in the art and will not be discussed in detail here. Briefly, ablation utilizes a laser to selectively melt the recording medium. The surface tension of the molten liquid pulls the film away from the center, resulting in the formation of a hole. Optical media on which data is recorded by a topography change include bubble forming media. A bubble is formed due to either gas evolution from the underlying polymer or the microswelling of the metal or polymer layer upon laser irradiation. In chemical reaction recording, a laser-induced chemical reaction is initiated between two initially discrete layers. In particle coalescence, a very thin and discontinuous metal film is subjected to laser irradiation to induce a coalescence of metal particles. In phase changes, the optical properties of the recording medium are changed by inducing phase changes therein. Depending on the laser characteristics, the recording medium exhibits two different reflectances and the change between the two states may be reversible. The reversible change may, for example, be an amorphous to crystalline phase transition. In magnetization, the area irradiated with the laser exhibits a changed direction of magnetization. A linearly polarized laser light of low intensity is used to sense the change in magnetic direction.
So-called image filing systems are widely used as business machines for recording and reproducing document data. In these image filing systems, image data is first optically read from a document, and is then recorded onto a recording medium. The image data recorded onto the recording medium may be read therefrom and subsequently reproduced on a display unit for visual presentation or supplied to a printer to produce a hard copy.
Optical disc devices used in these image filing systems employ optical discs to record and store the image data. Image data is recorded in spiral tracks on the surface of the optical disc. An optical head records and reads out image data on the optical disc. The optical head is positioned close to the optical disc and is driven by a linear motor so as to rectilinearly move in the radial direction of the optical disc.
Two methods have been developed to record and read image data on an optical disc. The first method is a so-called constant linear velocity (CLV) method, and the second method is a so-called constant angular velocity (CAV) method. In the CLV method, the optical disc is rotated such that the angular velocity of the disc is decreased as the optical head is moved radially outward from the center portion to the peripheral portion of the optical disc. The CLV method ensures that all tracks on the optical disc move at a constant speed relative to the optical head.
Thus, in the CLV method the angular velocity of the optical disc changes with the radial position of the optical head as the head is moved above the surface of the optical disc. However, in order to access image data on the disc, the angular velocity of the disc must reach a constant value which requires that a relatively long period of time elapse. Thus, the CLV method requires a long access time and exhibits a slow data transfer speed.
In the CAV method, the angular velocity of the optical disc is maintained at a constant value in order to stabilize the recording and reading operations and to reduce access time. However, since the angular velocity of the optical disc is constant, image data becomes less dense with increasing radius. Thus, the CAV method does not lend itself to the production of high density optical discs.
Some improvements have been proposed for the CLV method. In one improvement, the angular velocity of the optical disc is set to a constant value. The frequency of a system clock is then varied in accordance with the position for recording and reading data such that the data are formed with a predetermined constant spacing therebetween along the tracks. Hereinafter, this method will be referred to as the Constant Linear Density (CLD) method.
In the CLD method, the frequency of the clock is increased as the optical head is moved radially outwardly. However, it becomes difficult to control the high clock frequency as the optical head is located adjacent the outermost radial portions of the disc and recording quality decreases.